1. Field of the Invention
The present invention relates generally to a catalytic converter for purifying an exhaust gas discharged from an internal combustion engine of a vehicle, and particularly to a producing method of a honeycomb-shaped catalyst carrier mainly made of cordierite.
2. Related Art
Conventionally, a cordierite honeycomb body made of cordierite and having a honeycomb shape is used as a catalyst carrier for carrying a catalyst for purifying an exhaust gas discharged from a vehicle engine. To produce the cordierite honeycomb body, source materials such as talc (Mg.sub.3 Si.sub.4 O.sub.10 (OH).sub.2), kaoline (Al.sub.2 Si.sub.2 O.sub.5 (OH).sub.4) and alumina (Al.sub.2 O.sub.3) are mixed according to a desired cordierite composition. The mixture is formed into a desired honeycomb shape, and is sintered to become a cordierite honeycomb body.
Recently, it is necessary for the catalyst carrier for a vehicle to reduce a weight thereof and a pressure loss of exhaust gas flowing therethrough. Thickness of a cell wall of a cordierite honeycomb body consisting of the catalyst carrier may be decreased to meet this demand. However, when thickness of the cell wall is decreased, strength of the cordierite honeycomb body is also lessened. Thus, density of cordierite forming the cordierite honeycomb body needs to be increased so that strength of the cordierite honeycomb body is maintained even when thickness of the cell wall is decreased. For example, when thickness of the cell wall is decreased to 100 .mu.m or less, porosity of cordierite forming the honeycomb body is preferably decreased to 20% or less so that the honeycomb body has sufficient strength.
It is known that cordierite material particles are finely grained or sintering aids are added to cordierite material so that density of cordierite is increased. However, these methods facilitate cordierite to be sintered, thereby increasing density of cordierite. Therefore, density of microcracks generated in cordierite and correlative with a coefficient of thermal expansion (hereinafter referred to as C.T.E.) of cordierite is also decreased. That is, in these methods, porosity of cordierite may be decreased, but the C.T.E. of cordierite may be increased. Thus, it is difficult to decrease porosity of cordierite so that density of cordierite is increased, while preventing the C.T.E. thereof from increasing. Further, when the cordierite honeycomb body having such a large C.T.E. is used as the catalyst carrier, a large thermal stress is generated by heat emitted while exhaust gas is purified by the catalyst carrier. As a result, cracks may occur in the catalyst carrier.